The present invention is directed to an apparatus and method for controlling the flatness or warping of sheet material. More particularly, one embodiment of the present invention utilizes infrared reflectance-type moisture sensors to measure the cross-directional moisture content profiles of the two outer paper sheet liners of a corrugated paperboard sheet. The device then controls the moisture content profiles of the liners based upon these moisture measurements, to thereby control the warping of the sheet.
The manufacture of corrugated paperboard sheet requires the bonding together of three components: a top liner, a corrugated medium and a bottom liner. The top and bottom liners are typically heavy sheets of paper (e.g., cardboard), and the corrugated medium is typically also sheet paper, but usually of lighter weight. In a typical manufacturing process, the corrugated medium is first bonded by gluing to the top liner. The bonded top liner and corrugated medium then pass through a heated chamber called a "single facer." The elevated temperature within the single facer dries the glue, thereby firmly securing the corrugated medium to the top liner. The bottom liner is then glued to the exposed side of the corrugated medium, and the three-component sheet passes through a second heated chamber, called a "double facer." The elevated temperature within the double facer dries the glue holding the corrugated medium to the bottom liner so that the three components of the corrugated paperboard sheet, i.e., the top and bottom liners and the corrugated medium, are now firmly bonded together.
Ideally, once these three components have been bonded together, the resulting corrugated paperboard sheet should then have, and thereafter continue to have, equal or balanced stresses within the top and bottom liners. Having such balanced stresses will result in a non-warped sheet, since the stresses in the top liner will exactly cancel the stresses in the bottom liner. However, without some external influence over the factors tending to create unbalanced stresses in the top and bottom liners, a warped sheet tends to be the rule rather than the exception.
One culprit commonly contributing to unequal stress in the top and bottom liners, and therefore to warped sheets, is a different "cross-directional" moisture content profile remaining in the top liner as compared to that in the bottom liner after the three components have been bonded together. (The "cross-directions" are defined as the two directions in the plane of the sheet perpendicular to sheet movement through the corrugated paperboard sheet-making machine. The direction of sheet movement is called the "machine direction.") Different cross-directional moisture content profiles as between the two liners will result when, for example, machine direction streaks of higher than average moisture content are present within different opposing longitudinal segments of the top liner as compared to the bottom liner. (Such longitudinal segments are referred to as "slices.") Alternatively, different cross-directional moisture content profiles may also result when streaks of different moisture contents are present within corresponding opposing slices of the two liners.
Differences in cross-directional moisture content profiles between the two liners can occur quite readily for a variety of reasons. For example, the paper sheet used for the top liner may have been manufactured with faulty or inadequately controlled equipment which allowed moisture streaks to remain in the finished products, thereby causing the two liners to have different cross-directional moisture content profiles. Regardless of the reason, however, once top and bottom liners having unbalanced cross-directional moisture content profiles are bonded together with the corrugated medium, and the resulting corrugated paperboard sheet dries, cross-directional warpage can occur. This is because the respective cross-directional moisture content profiles of the liners affect cross-directional shrinkage that takes place in each liner during drying. Therefore, with uneven cross-directional shrinkage, the corrugated paperboard sheet may tend to curl into a long tube after it comes out of the corrugated sheet manufacturing machine.
Drying one liner or the other prior to bonding has been used in an attempt to prevent warping of the resulting board in the machine direction. Manufacturers have installed steam heated drying drums in certain corrugated paperboard sheet manufacturing machines. Prior to being bonded to the corrugated medium, the top and bottom liners are passed through separately controllable sets of these drying drums. The resulting corrugated sheet is watched, by eye, for any warping. Then, if the finished manufactured sheet curves upward toward the top liner, the steam supply is increased to the drying drums associated with the bottom liner, thereby decreasing the moisture content of this liner. Conversely, if the sheet curves toward the bottom liner, then the steam supply is increased to the drying drums associated with top liner, thereby decreasing the moisture content of the top liner. The goal is to obtain top and bottom liners having the same average moisture content. Unfortunately, however, differences in cross-directional moisture content profiles between the top and bottom liners cannot be corrected using steam-filled drying drums since such drums heat the liners equally across their entire widths.
Corrugated paperboard sheet manufacturers have also installed water spray nozzles on the corrugated paperboard manufacturing machines in an attempt to prevent, or at least minimize, warping of the resulting paperboard in the cross-direction. With this technique, a plurality of manually controlled water spray nozzles are disposed at intervals across each liner as the liners are being fed into the corrugated paperboard manufacturing machine. The machine operator then watches the paperboard as it is being produced and manually adjusts the pattern of water sprayed onto each liner from the nozzles in an attempt to flatten the resulting paperboard sheet. Unfortunately, however, this technique is time consuming even with a highly skilled and experienced operator. While the water spray pattern is being manually adjusted, large quantities of substandard, warped paperboard sheet may be produced. Also, unbalanced water content as between the two opposing liners is not the only factor which may produce warping. Thus, even if a skilled operator could produce an unwarped sheet, the water spray pattern necessary to produce such a sheet may mask other problems in the manufacturing process.
Finally, differences in moisture content between the top and bottom liners does not always warp the resulting corrugated paperboard sheet until sometime after the sheet is manufactured. Accordingly, large quantities of corrugated paperboard sheet may be manufactured before the unbalanced cross-direction and/or machine direction moisture content profiles become apparent through visually detectable warpage of the sheet.